A loudspeaker is a device that changes electrical signals into audible sounds. Its design is an important determinant of overall performance of an audio reproduction system. In choosing a particular loudspeaker design, engineers balance many competing considerations. Such considerations include frequency range of the loudspeaker, in-band amplitude and phase distortions, efficiency, and the so-called “Q” factor. The following paragraphs briefly discuss these considerations.
The frequency range of the loudspeaker should cover at least some portion of the audible frequency band, which extends from about 20 Hz to about 20 KHz. Generally, the wider the frequency range of the loudspeaker within the audible frequency band, the better. Because of the difficulty of designing high-quality speakers covering broad frequency ranges, some systems employ dedicated loudspeakers for reproduction of the low-end frequencies, in addition to other loudspeakers used for reproduction of mid-range and higher frequencies. The dedicated low-end loudspeakers, often referred to as woofers or subwoofers, typically cover the frequency range of between about 20 Hz and about 120 Hz.
Distortion means unwanted alteration of a waveform. Therefore, both phase distortion and amplitude distortion (also known as ripple), should be minimized to reproduce the original sound more authentically.
Efficiency is the ratio of the acoustic energy generated and radiated by the loudspeaker to the total electric energy delivered to the loudspeaker. Maximizing loudspeaker efficiency is important for several reasons. First, the higher is the efficiency, the lower is the required output power rating of the amplifier (or another source) driving the loudspeaker. Second, the power that is not radiated is converted into heat, which has to be removed from the loudspeaker, lest the loudspeaker overheat. And, of course, the consumption of the electric power by itself can be an important design factor, particularly for portable audio systems.
The Q factor is the ratio of the reactance and resistance of the electrical circuit model of the loudspeaker. Many loudspeakers operate with the Q factor in the range from about 0.2 to about 1.2. Musical speakers typically have the Q factor of about 0.6–0.7, while more accurate or “tight” speakers have the Q factor approaching 1.0–1.1. The Q factor range of about 0.2 to about 1.2 is rather subjective, but generally provides a relatively flat response curve. In contrast, other loudspeakers operate with higher Q factors. Their efficiencies are lower and their sound is typically more “booming” and distorted.
A typical dynamic loudspeaker includes an electrodynamic motor and a diaphragm, also known as a cone. The motor of the loudspeaker includes wire or voice coil windings on a former. The coil windings and the former slide along a cylindrical pole piece in a magnetic field generated by a permanent magnet. The former is mechanically coupled to the diaphragm. When an electrical current flows through the voice coil, the coil moves under influence of the Lorentz electromotive force exerted by the magnetic field of the permanent magnet on the charged particles flowing in the windings of the voice coil. The diaphragm moves together with the coil, creating variable acoustic pressure that reproduces the sound represented by the current.
The efficiency of the dynamic loudspeaker with a moving voice coil is low for at least two reasons. First, the movement of the diaphragm “pushes out” the air on one side (e.g., the front), while “pulling in” the air on the opposite side (e.g., the back). The two movements tend to cancel each other, unless the loudspeaker is placed within an enclosure. When the loudspeaker is placed in an enclosure, the movement of the diaphragm increases and decreases the volume within the enclosure, corresponding to the movement of the diaphragm out and into the enclosure, respectively. The changes in the volume of the enclosure generate changes in the air pressure within the enclosure, which must be counteracted by the diaphragm. This condition exists in both sealed and vented enclosures, and creates an additional load on the diaphragm and on the motor. The additional load consumes energy and lowers the efficiency of the loudspeaker.
Second, air density is low. Therefore, the voice coil needs to drive a large diaphragm surface at a high velocity to radiate significant acoustical pressures. The structural integrity required by a large, fast moving diaphragm necessitates a sturdy construction of the diaphragm and its supporting structure. The combined mass of the diaphragm and the supporting structure is large in comparison to the mass of the air moved. Essentially, a heavy diaphragm must be moved to push a small mass of air. In technical terms, the acoustic impedance of the diaphragm is much higher than the impedance presented by the moving air.
For a fixed loudspeaker enclosure volume, efficiency increases with the increase in the low corner cutoff frequency (fc) of the loudspeaker. This relationship is known as Hoffman's Iron Law. Stating this law differently, for a given volume of the enclosure, increasing efficiency will generally increase the low corner cutoff frequency fc of the loudspeaker, diminishing the loudspeaker's low frequency response.
Increasing loudspeaker efficiency also decreases the Q factor of the loudspeaker. Recall that a decrease in the Q factor may make the loudspeaker less accurate.
An increase in loudspeaker efficiency can thus entail a performance penalty, particularly when it is achieved without a corresponding increase in the volume of the loudspeaker's enclosure. Moreover, efficiency is not the end all and be all of the loudspeaker design; high efficiency may not even be needed in some applications. For example, an amplifier driving the loudspeaker may have the capacity to drive a low-efficiency loudspeaker with a signal sufficient to reproduce sound with the required volume, and the installed environment of the loudspeaker may provide abundant ventilation for cooling. In this case, loudspeaker efficiency can be sacrificed to obtain a better low frequency response and more authentic sound reproduction capability of the audio system. Conversely, performance may have to be sacrificed for the sake of efficiency where a predetermined sound level has to be obtained from a relatively weak amplifier/driver, especially in a small enclosure. It follows that a loudspeaker with fixed design parameters—including efficiency—may not be the optimum device for a particular system. In fact, such a loudspeaker may not even provide the minimum acceptable performance level required by the system.
Sound preferences are no less subjective than beauty which, according to a well-known expression, resides in the eye of the beholder. Some listeners prefer “tight” loudspeakers, while others favor musical loudspeakers. The ability to tune the sound of an audio system, beyond simple treble, bass, and other equalizer adjustments, would be a valuable feature of a loudspeaker.
Vendors of loudspeakers, and particularly of subwoofers, often require custom-made enclosures to match the parameters of the loudspeaker motor structure. (A motor structure may include a voice coil, magnet, diaphragm, and related components.) It would be desirable to be able to match the motor structure of a loudspeaker to a range of enclosures, rather than limiting the motor structure to a custom-made enclosure.
A need thus exists for a loudspeaker that can be adapted to various installed environments. A further need exists for a loudspeaker that can be customized for installations within enclosures of various sizes. A still further need exists for a loudspeaker with adjustable sound reproduction characteristics.